The invention relates to a raw material with a high content of aluminum oxide (also referred to herein as a high-alumina raw material), wherein the aluminum oxide Al2O3 is present in the mineralogical form of corundum α-Al2O3, spinel MgAl2O4, aluminum mono-hydroxide Al2O3.H2O and aluminum tri-hydroxide Al2O3.3H2O, and wherein the weight ratio of aluminum mono-hydroxide to aluminum tri-hydroxide is larger than 0.25. Moreover the invention relates to a process of producing the high-alumina raw material starting from an alumina product extracted from aluminum salt slags, and it also relates to the use of the high-alumina raw material as a sinter-active alumina source for producing ceramic and refractory materials, cement, porosified binding agents used in the construction industry, slag formers for the production of iron and steel, mineral wool and ceramic fibers.
High-alumina raw materials with an Al2O3 content of 50 to 80 percent are available either as natural raw materials (such as bauxite, sillimanite, kyanite and andalusite) or are synthetically produced by chemical processing of bauxite into pure alumina followed by sintering or melting (as is the case for mullite and spinel). In the interest of conserving natural resources, there is a growing trend towards using secondary raw materials that are recovered from waste materials which occur as by-products of recycling processes.
Aluminum salt slag is generated as a by-product in the recycling of aluminum metal. In this process, the surface of the molten metal is covered with a layer of salt (for example a mixture of 70 percent NaCl, 28 percent KCl and 2 percent CaF2). The purpose of the salt layer is to minimize the oxidation of the metal and to absorb oxidic impurities in the form of a salt slag. The salt slag contains about 40–50 percent salt, 40–50 percent oxides and 10 percent metallic aluminum. As aluminum during melting in air does not only combine with oxygen but also with nitrogen, a salt slag also contains aluminum nitride in addition to aluminum oxide.
In order to recover the components as completely as possible, the salt slag is subjected to a treatment in which most of the metal can be mechanically recovered after crushing the salt slag. Subsequently, the salt slag is treated with water and the salt is dissolved. After separating the insoluble components, herein referred to as “alumina product”, the salt is crystallized by evaporating the solution and is thus made available again for the aluminum-melting process described above.
The alumina product contains about 50–80 percent Al2O3. According to M. Beckmann (Aluminum 67 [1991] 586–593), the Al2O3 is present in the mineralogical form of corundum α-Al2O3, spinel MgAl2O4 and aluminum tri-hydroxide β-Al2O3.3H2O (bayerite). Aluminum tri-hydroxide is formed in the wet treatment of salt slag by the reaction of water with aluminum or aluminum nitride according to the following equations:2Al+6H2O=Al2O3.3H2O+3H22AlN+6H2O=Al2O3.3H2O+2NH3
In a practical wet treatment process, the particle size to which the salt slag is crushed and the reaction time are set according to economic criteria with the result that the alumina product can have a residual content of as much as 5 percent aluminum metal and aluminum nitride.
The alumina product is taken out of the wet treatment process as a sludge filtrate in cake form (in most cases with a water content of about 30–45 percent, not in a suitable form for transporting or for measuring out controlled quantities, and with a strong odor of ammonia. The unfavorable consistency and the residual content of aluminum metal and aluminum nitride are a considerable drawback associated with using the alumina product, particularly for producing sintered or fused materials. In the further processing of the alumina product (mixing with other mineral materials, drying, calcining, sintering, melting) the following difficulties occur, some of which represent major problems:                Exposure of workers to malodorous ammonia vapors during handling;        Strong thermal reaction and release of gas (water vapor, H2, NH3) when mixing the product with quicklime and cement, resulting in poor mechanical strength of molded bodies (for example pellets and briquettes);        Corrosion of equipment caused by release of gas;        Risk of explosion caused by release of gas;        Accumulations attaching themselves and forming mechanical obstructions in the machinery and transport paths;        Difficult to control local temperature variations, specifically overheating of rotary kilns used in sintering processes due to uncontrolled thermal reactions of the aluminum metal and the aluminum nitride, resulting in undesirable melting, baked-on material accumulations and formation of lumps.        
The reference DE-4319163 discloses a fine-grained filler material for cement-bound solid materials, a residual substance of high-alumina content (i.e., an alumina product) generated in the processing of salt slags in the aluminum industry, which—relative to its dry weight—contains 55–70 percent Al2O3, 7–10 percent MgO, 6–9 percent SiO2 and a loss on ignition of 7–11 percent. No information is presented on aluminum hydroxide content.
The reference DE-4345368 describes a process for producing sintered spinel containing aluminate cement clinker based on the same residual material as in DE-4319163, at sintering temperatures above 1100° C.
The reference EP-0838443 proposes a process in which aluminum-containing waste materials are used in the production of sintered sulfo-aluminate cement. The process is fed, for example, with a by-product from the treatment of aluminum salt slags, containing 65.7 percent Al2O3, 10.1 percent MgO, 7.9 percent SiO2 and 7.3 percent H2O, in which the Al2O3 is present in the crystalline phases of corundum α-Al2O3, bayerite Al(OH)3 and spinel MgAl2O4. The sintering temperatures are in the range of 1150–1300° C. (examples 1 and 2).
The reference EP-0881200 describes a refractory bottom lining for aluminum electrolytic cells in the form of a monolithic ramming mix of dried and calcined formed bodies (pellets, briquettes, bricks) based on a powder with high-alumina content resulting from the treatment of dross and salt slags in the aluminum industry, containing 40–90 percent Al2O3, 4–20 percent MgO, 0.5–15 percent SiO2, 0.1–25 percent AlN, 0.1–10 percent Al (metallic) and a maximum of 10 percent loss on ignition. Crystalline phases contained in the powder include corundum α-Al2O3, spinel MgAl2O3 and fluorspar CaF2. Aluminum hydroxide is not mentioned.
In EP-1036044, a process for producing synthetic glass fibers (rock wool, mineral wool) is disclosed, wherein 2–20 weight-percent of the mineral solids are mineral waste with a content of halogens of at least 1 weight-percent, such as alumina residues from the treatment of aluminum dross and aluminum salt slag, with a content of 0.5–10 weight-percent aluminum metal, 50–90 weight-percent Al2O3, and up to 5 weight-percent fluorine or other halogens. This reference contains no information on contents of aluminumhydroxide.
EP-1180504 discloses a porosifying, solidification-accelerant additive for binding agents in the form of a high-alumina powder, containing 50–90 percent Al2O3, up to 10 percent Aluminum (metallic) and main mineral constituents in the form of corundum (α-Al2O3) and spinel (MgAl2O4), wherein the metallic aluminum particles are enveloped by mineral transition modifications of aluminum tri-hydroxide (Al2O3×3H2O) to α-alumina. According to the description of the process of producing the powder, a high-alumina waste material from the treatment of aluminum salt slag is shock-dried at flue gas temperatures from 400 to 500° C. and subsequently partially calcined at temperatures up to 1000° C. According to C. Misra (Industrial Alumina Chemicals, American Chemical Society 1986, page 76), no transformation into aluminum mono-hydroxide occurs with a rapid thermal dehydration of aluminum tri-hydroxides, but a direct transformation to the transition modifications ρ-Al2O3, η-Al2O3 (above 400° C.) and θ-Al2O3 (above 750° C.) Corundum α-Al2O3 occurs above 1200° C.
U.S. Pat. No. 5,045,506 describes the use of alumina products from the processing of aluminum dross and salt slag in the production of mineral wool, wherein (claim 6) the alumina product from the processing of salt slag must be heated (calcined) to a sufficiently high temperature, that the aluminum hydroxide in the alumina product is transformed to aluminum oxide.
U.S. Pat. No. 5,424,260 describes an alumina product NMP from the processing of aluminum salt slag and its use for the production of ceramic fibers. The alumina product NMP contains 40–75 weight-percent Al2O3, 5–20 weight-percent MgO, 2–15 weight-percent SiO2, with a loss on ignition of about 20 weight-percent (column 5, lines 57–68). There is no information on aluminum hydroxide content.
U.S. Pat. No. 6,238,633 discloses a process for producing a sintered calcium aluminate slag former for use in the production of steel, based on a non-metallic product NMP (column 5, lines 44 ff.), consisting of 40–75 weight-percent Al2O3, 0–20 weight-percent MgO, 2–15 weight-percent SiO2, in each case less than 1 percent aluminum metal and aluminum nitride, and a loss on ignition at 1400° C. of 5–35 weight-percent. No information on the contents of aluminum hydroxides is given other than a reference to an “amorphous alumina precursor phase” or “hydrated amorphous phase” that is transformed into corundum during calcining (column 6, line 39–48). After mixing with CaO or CaO-carriers (calcium hydroxide or calcium carbonate), pellets or extruded shapes are formed and heated to a temperature between 1093 and 1193° C. A calcium aluminate produced in this manner has a melting point of 1360° C. (column 12, line 6–8). The NMP is produced from aluminum salt slag, wherein a reduction of the contents of aluminum metal and aluminum nitride by a fine sieve separation helps to prevent materials from sticking to surfaces of agglomerating into lumps during the sintering of the calcium aluminate (column 11, line 46–49). The disadvantage of this process is, that the mechanically separated aluminum metal and aluminum nitride are no longer available for the formation of aluminum oxide. Thus, the Al2O3 content of the alumina source is reduced, a higher total quantity of the alumina source is needed in the mixture, and the impurities in the NMP lead to a loss of purity and degradation of quality in the calcium aluminate.